The present invention relates to a control system for a load sensing hydraulic drive circuit used in hydraulic machines such as hydraulic excavators or cranes, and more particularly to a control system for a load sensing hydraulic drive circuit equipped with pump control means which controls a delivery pressure of a hydraulic pump so as to hold it higher by a predetermined value than a load pressure of a hydraulic actuator.
Hydraulic drive circuits for use in hydraulic machines such as hydraulic excavators or cranes each comprise at least one hydraulic pump, at least one hydraulic actuator driven by a hydraulic fluid delivered from the hydraulic pump, and a flow control valve connected between the hydraulic pump and the actuator for controlling a flow rate of the hydraulic fluid supplied to the actuator. It is known that some of those hydraulic drive circuits employs a technique called load sensing control (LS control) for controlling a delivery rate of the hydraulic pump (thereby constituting an LS regulator). The LS control is to control the delivery rate of the hydraulic pump such that the delivery pressure of the hydraulic pump is held higher by a predetermined value than the load pressure of the hydraulic actuator. This causes the delivery rate of the hydraulic pump to be controlled dependent on the load pressured of the hydraulic actuator, and thus permits economic operation. Also, connected to a delivery line of the hydraulic pump is an unloading valve for holding a differential pressure between the delivery pressure of the hydraulic pump and a maximum load pressure among the actuators less than a setting value.
Meanwhile, the LS control is carried out by detecting a differential pressure (LS differential pressure) between the delivery pressure and the load pressure, and controlling the displacement volume of the hydraulic pump, or the position (tilting amount) of a swash plate in the case of a swash plate pump, in response to a deviation between the LS differential pressure and a differential pressure target value. To date, the detection of the differential pressure and the control of tilting amount of the swash plate have usually been carried out in a hydraulic manner as disclosed in U.S. Pat. No. 4,617,854 (corresponding to DE, A1, 3422165), for example. This conventional arrangement will briefly be described below.
An LS regulator disclosed in JP, A, 60-11706 comprises a control valve having one end subjected to a delivery pressure of a hydraulic pump and the other end subjected to both a maximum load pressure among a plurality of actuators and an urging force of a spring, and a cylinder unit operation of which is controlled by a hydraulic fluid passing through the control valve for regulating the swash plate position of the hydraulic pump. The spring at one end of the control valve is to set a target value of the LS differential pressure. Depending on a deviation occurred between the LS differential pressure and the target value thereof, the control valve is driven and the cylinder unit is operated to regulate the swash plate position, whereby the pump delivery rate is controlled so that the LS differential pressure is held at the target value. The cylinder unit has a spring built therein to apply an urging force in opposite relation to the direction in which the cylinder unit is driven upon inflow of the hydraulic fluid.
In the above LS regulator, a tilting speed of the swash plate of the hydraulic pump is determined by a flow rate of the hydraulic fluid flowing into the cylinder unit, while the flow rate of the hydraulic fluid is determined by both an opening, i.e., an position, of the control valve and the setting of the spring in the cylinder unit. The position opf the control valve is, in turn, determined by the relative relationship between the urging force of the LS differential pressure and the spring force for setting the target value of the differential pressure. Here, the spring in the control valve and the spring in the cylinder unit have their specific spring constants. Accordingly, a control gain for the tilting speed of the swash plate dependent on the deviation between the LS differential pressure and the target value thereof is always constant.
On the other hand, the unloading valve is generally operated in response to a signal indicative of the difference between the delivery pressure of the hydraulic pump and the maximum load pressure among the actuators, such that when the LS differential pressure exceeds a setting value of a spring disposed in the unloading valve for such reason as a response delay of the LS regulator, the hydraulic fluid in the delivery line of the hydraulic pump is discharged to a reservoir through the unloading valve, thereby maintaining the preset differential pressure in a quick manner. Usually, the preset differential pressure of the spring in the unloading valve is selected to be slightly higher than the preset differential pressure of the spring in the LS regulator's control valve.
However, the above conventional control system for the load sensing hydraulic drive circuit has suffered from problems below.
The LS regulator is intended to, as stated above, control the swash plate position dependent on the signal indicative of the difference between the delivery pressure of the hydraulic pump and the maximum load pressure among the actuators, thereby holding the LS differential pressure at the setting value of the spring in the control valve. During the LS control, when an operation (input) amount (i.e., a demanded flow rate) of the flow control valve is small and so is an opening of the flow control valve, the delivery pressure of the hydraulic pump is substantially determined by a difference between the flow rate flowing into a line, extending from the hydraulic pump to the flow control valve, and the flow rate flowing out of the line, as well as the volume modulus of the line. The volume modulus of the line is given by dividing the volue modulus of the hydraulic fluid (oil) by the volume of the line. Since the volume of the line is very small, the volume modulus of the line takes a large value as the opening of the flow control valve is small. Even with slight change in the flow rate, therefore, the delivery pressure is so greatly changed as to cause a hunting and thus render the control of the LS differential pressure difficult.
On the contrary, when the operation amount of the flow control valve is increased to enlarge the opening thereof, the circuit into which the delivery rate of the hydraulic pump flows now takes the large volume including a cylinder, resultig in the smaller volume modulus. Therefore, change in the delivery pressure upon change in the delivery rate of the hydraulic pump is reduced, making it easy to carry out the control of the LS differential pressure.
Accordingly, in order to reliably perform the control of the LS differential pressure over a range of the entire operation amount of the flow control valve, it is required to allow easy implementation of the control of the LS differential pressure when the opening of the flow control valve is small. This could be achieved by setting the control gain of the LS regulator, i.e., the setting values of the aforesaid two springs such that the changing or tilting speed of the swash plate of the hydraulic pump becomes slow. However, if the control gain is so set, there would arise another problem that when the opening of the flow control valve is large, the volume modulus is reduced as mentioned before, which also reduces a change rate of the LS differential pressure and thus degrades a response of the LS control.
In addition, there is also known a control system in which a pump of fixed displacement volume type is used as the hydraulic pump, and unloading valve is connected to a delivery line of the pump, and the differential pressure between the pump delivery pressure and the maximum load pressure among the actuators under the action of the unloading valve only. One of this type control system is disclosed in U.S. Pat. No. 3,976,097, for example.
An object of the present invention is to provide a control system for a load sensing hydraulic drive circuit for controlling a pump delivery rate, which can realize stable control of the LS differential pressure with small pressure change even when the operation amount of a flow control valve is small, and which can also control the hydraulic pump with a quick response when the operation amount of the flow control valve is large.